A Stable Isotopic Study Of Seawater Interaction With The Lower Ocean Crust, ODP Hole 735B

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Science Summary: Under this award, the PIs propose to study oxygen and hydrogen isotopes in samples from the deepest one km. of core from ODP site 735B on Atlantis Bank, the deepest penetration of seafloor gabbros in existence. The PIs will also analyze strontium isotopes in vein minerals and whole rocks to complement the stable isotope measurements. The purpose of these analyses is to determine whether the bulk gabbro section is depleted in 18O, as expected in models in which seawater d18O is buffered by hydrothermal activity, or whether instead the bulk gabbro is enriched in 18O as it is in some inferred slow-spreading ophiolites. A final possibility is that the signal of seawater interaction diminishes with depth in the crust as observed in the Troodos ophiolite and it is this particular hypothesis that the PIs will try to test. The bulk d18O values will have implications for mantle effects after subduction of 735B-like materials. A second important objective is to determine to what extent seawater was involved in high temperature deformation zones in the 735B gabbros, and whether late stage magmatic fluids (oxide-rich and/or silica-rich) were also involved in the deformation. In addition, the PIs will evaluate the extent to which brittle faults served as major conduits for fluids. This part of the study has important implications for fluid pathways into the deep ocean crust. A final objective of the study is to measure the d18O, D/H, and strontium isotope values of secondary mineral veins and felsic veins in the lower one km of the core. This is important because these veins affect the bulk composition of the section, and also because the veins document the interaction between the lower crust and circulating seawater and late magmatic fluids.